Steam-turbine type power plants require large fossil-fired steam generators for producing the steam to be expanded in the turbines which drive the electrical generators. In addition, such plants require superheaters, piping and valves all of which components must be brought up to suitable temperatures, or at least a minimum temperature, before the turbines can be started. In conventional steam-turbine power plants, the heat required for startup in order to reach the minimum temperature is supplied by the main steam generator. This is accomplished by operating the main steam generator, in a startup mode to be further described, and utilizing the heat of the steam by passing it through the superheater, main steam lines and then to a condenser.
This method of startup has several severe disadvantages such as, for example, the fact that the steam gives up more heat to the condenser than to the steam piping which constitutes a significant waste of energy. Also, the various components of the power plant are not heated to individually ideal temperatures.
In addition to being wasteful and inefficient, the use of the main steam generator to provide the heat of startup produces significantly increased production rates of nitrogen oxide on a pound per BTU basis. That is, nitrogen oxide production rates are substantially higher than those produced during normal, steady-state operation of the power plant. This is caused by the fact that the firing rate of the main generator during startup must be held relatively low so as not to exceed temperature limits of the metals, or rates of temperature change, or maximum temperature differentials of the components. However, safety considerations require that the flow of combustion air must be at least twenty-five percent of full load flow any time that the furnace is fired. Thus, during startup conditions, there is an excessive amount of oxygen in the oxygen-fuel ratio, and the presence of such excessive amounts of oxygen above the stoichiometric amount results in significantly increased amounts of nitrogen oxides being produced.